With 21 states having adopted bans or severe restrictions on insurance companies from paying for abortions, Washington is alone in seriously considering legislation mandating the opposite.

The Reproductive Parity Act, as supporters call it, would require insurers in Washington state who cover maternity care — which all insurers must do — to also pay for abortions.

The bill passed the state House earlier this month by a vote of 53 to 43, though it faces an uncertain future in the Senate.

The proximate cause of the measure is the federal Affordable Care Act. Thanks to language in it pushed by Republicans, insurers selling their plans on the state exchanges taking effect next year will have to segregate the premiums they collect for abortion coverage.

In addition to that built-in disincentive to insuring abortion, the law also invites states to enact stricter rules of their own. Thus far, 16 states have followed suit, barring or restricting insurance companies on their exchanges from covering the procedure.

Supporters of Washington state’s proposed abortion insurance mandate stress that it would not lead to a dramatic uptick in abortions or require carriers with a religious bent to cover the procedure. They also note that a pair of federal plans that will be sold on all 50 state exchanges will be barred from covering elective abortions.

‘‘It’s not expanding abortion coverage,’’ said Democratic Representative Eileen Cody,the bill’s primary sponsor. ‘‘It’s ensuring the rights of women to get what they’re paying for now and to continue their freedom of choice.’’

Opponents counter that the measure would require businesses and individuals to pay for abortion coverage they would rather not have.